英文:
Why is this type assertion from bson.M necessary?
问题
我有以下代码，使用了go.mongodb.org/mongo-driver库：
updateMap := bson.M{
"$set": bson.M{
"dateAdded": primitive.NewDateTimeFromTime(time.Now()),
},
}
if len(alias) > 0 {
// 我不明白为什么需要这个类型断言
updateMap["$set"].(map[string]interface{})["alias"] = alias
}
如果我这样写：
updateMap["$addFields"].(map[string]map[string]string)["alias"] = alias
它无法编译通过。编译器已经知道updateMap的类型是bson.M，它是一个map[string]interface{}。那么为什么要断言它真的是一个map[string]interface{}对编译器有用呢？我以为它想知道它实际上是什么类型的接口，但显然它不希望我这样做。
这里发生了什么？
编辑：当然，需要注意的是，如果没有类型断言，它无法编译通过：
invalid operation: cannot index updateMap["$addFields"] (map index expression of type interface{})
英文:
I have the following code, making use of go.mongodb.org/mongo-driver libraries:
updateMap := bson.M{
"$set": bson.M{
"dateAdded": primitive.NewDateTimeFromTime(time.Now()),
},
}
if len(alias) > 0 {
// I don't see how this type assertion is necessary
updateMap["$set"].(map[string]interface{})["alias"] = alias
}
If I do:
updateMap["$addFields"].(map[string]map[string]string)["alias"] = alias
it doesn't compile. The compiler already knows that updateMap is of type bson.M which is a map[string]interface{}. How does asserting that it really is a map[string]interface{} doing anything useful for the compiler? I thought it would want to know what kind of interface it actually is, but apparently it doesn't want me to do that.
What is going on here?
edit: and of course, it should be noted that without a type assertion here, it doesn't compile:
> invalid operation: cannot index updateMap["$addFields"] (map index expression of type interface{})
答案1
得分: 1
bson.M被定义为primitive.M的别名，而primitive.M被定义为type M map[string]interface{}（即非别名类型）。问题在于updateMap["$set"]是一个bson.M，与map[string]interface{}相比是一个不同的类型。因此，你的类型断言应该是bson.M，例如：
updateMap["$set"].(bson.M)["alias"] = alias
英文:
bson.M is defined as an alias of primitive.M, which is defined as type M map[string]interface{} (that is, a non-alias type). The problem is that updateMap["$set"] is a bson.M, which is a distinct type compared to map[string]interface{}. So your type assertion would have to be to bson.M instead, for example:
updateMap["$set"].(bson.M)["alias"] = alias
通过集体智慧和协作来改善编程学习和解决问题的方式。致力于成为全球开发者共同参与的知识库，让每个人都能够通过互相帮助和分享经验来进步。
评论